Radar systems utilize waveguide circulators to route incoming and outgoing signals between an antenna, a transmitter and a receiver. Referring to FIG. 1(a), there is a schematic diagram of a dual junction conventional four port circulator 10, which has a first port 12 coupled to a transmitter, a second port 14 coupled to an antenna, a third port 16 coupled to a receiver and a fourth port 17 terminated by a matched load. The circulator 10 routes outgoing signals 13 from the transmitter (e.g. the first port 12) to the antenna (e.g. the second port 14) while isolating the receiver (e.g. the third port 16). Similarly, the circulator 10 routes incoming signals 15 from the antenna (e.g. the second port 14) to the receiver (e.g. the third port 16), while isolating the transmitter (e.g. the first port 12). The circulator routes incoming signals 15 and outgoing signals 13 concurrently (i.e. such that the antenna can transmit and receive signals at the same time). It is to be noted that during the time the transmitter is active (the transmission of a high power RF pulse), the residual power reflected by the antenna is high enough to trigger the receiver protector 19, FIG. 1(b). In this case, the circulator junction directly connected to the antenna will have to operate with full reflection at port 16. This is due to receiver protector properties known to those skilled in the art. Also, the circulator must properly operate in the event of excessive antenna reflected power (a failure mode). This last requirement implies that the circulator junction design must be done for a much higher peak RF power than the actual transmitter power.
Waveguide junction circulators are generally designed using one of the junction configurations presented in FIGS. 2(a) to 2(c). They are equal-ripple Chebyshev designs using partial height ferrite geometries between metal quarter wave transformer plates.
The first configuration shown in FIG. 2(a) is reserved for low power circulators and will not be discussed here. The second configuration shown in FIG. 2(b) is the basis of prior art commercial waveguide designs. This approach uses two identical ferrites in direct contact with the metallic walls. It is noted that the ferrite height marked as “L” in FIGS. 2(a) to 2(c) is not the same for the different configurations.
Referring to configuration shown in FIG. 2(b), in order to obtain the theoretical circulation conditions required, the gap between the ferrites becomes very small, as an example, around 0.2 inches (5 mm) for a quarter height L-band design. This is also due to the fact that the spacing between the two ferrites not only determine the phase angle of one eigennetwork but also the turn ratio of the ideal transformers used to represent the coupling of the two counter-rotating modes into the ferrite disks and the admittance of the radial quarter wave transformers as indicated in “Design data for Radial-Waveguide Circulators using Partial Height Ferrite resonators”, J. Helszajn, F. C. Tan, IEEE Trans. on MTT, vol-23, no. 3, March 1975. This particular aspect limits the maximum peak RF power which circulators designed according to the configuration shown in FIG. 2(b) can withstand without breakdown.
High power Radar Systems require circulators that operate not only at high RF peak power, but also at high average RF power due to the high duty cycle used by such systems. Since a microwave ferrite is a poor thermal conductor, a second problem appears, due to the fact that the configuration shown in FIG. 2(b) requires a relatively large ferrite diameter. Extreme mechanical stress of the ferrite disks appears due to the large thermal gradient generated by the uneven distribution of magnetic loss across the ferrite volume. This problem is in fact a potential failure mode of FIG. 2 (b) configuration and has manifested itself by circulator self-destruction.
Some circulators have been designed to improve performance at high power ratings. For example, U.S. Pat. No. 3,246,262 (Wichert) discloses a device for conducting heat away from a pre-magnetized microwave ferrite using a dielectric material arranged between the ferrite and a hollow conductor. According to one embodiment, Wichert discloses a ferrite body having a triangular cross-section and a longitudinal bore filled with a thermally conductive dielectric material that is in good contact with the ferrite and the hollow conductor. The dielectric material is a good conductor of heat, such as beryllium oxide, and removes heat produced in the ferrite. According to another embodiment, Wichert discloses three cylindrical ferrite bodies positioned so that they mutually touch each other. A hollow space in the center between the ferrite bodies is filled with a thermally conductive dielectric material for removing heat.
One problem with the circulators of Wichert is that the dielectric material removes a large portion of the ferrite from the center of the ferrite junction. Accordingly, the magnetic field tends to have a limited interaction with the ferrite junction, which tends to decrease performance and the circulator may have a limited bandwidth.
Another device is disclosed in United States Patent Application Publication No. 2007/139131 (Kroening). Kroening discloses an improved geometry for ferrite circulators that increases the average power handling by decreasing the temperature rise in the ferrite and associated adhesive bonds. The circulator includes thin dielectric attachments on the sides of the ferrite element, which maximizes the area of contact and minimizes the path length from the ferrite element out to the thermally conductive attachments. The dielectric attachments are made from good thermal conductors, such as boron nitride, aluminum nitride or beryllium oxide, which enables the dielectric attachments to be relatively thin. According to Kroening, these thin dielectric attachments minimize dielectric loading effects without impacting thermal performance.
One problem with the Kroening circulator is that the dielectric attachments are located on the outside of the ferrite element, which provides limited benefits because most of the heat is generated near the center of the ferrite junction due to more significant interactions between the ferrite junction and the magnetic field.
Accordingly, there is a need for improved high power waveguide circulators, and in particular, for improved high peak/average power waveguide circulators for use in Radar Systems.